1. Field of the Invention
The present invention relates in general to a control system for a variable displacement hydraulic pump, and more particularly to a feedback control system for a variable displacement hydraulic pump which is provided with a two-lever type of feedback lever mechanism and in which a horsepower control ram, displacing in response to a horsepower control signal, and a flow control ram, displacing in response to a flow control signal, are coaxially arranged, thereby controlling both the horsepower and the flow rate of the variable displacement hydraulic pump with a simple construction.
2. Description of the Prior Art
In order to control flow rate and horse power of variable displacement hydraulic pumps, especially for generating hydraulic power, there have been proposed several types of feedback control systems such as depicted in FIG. 1. This drawing shows a schematic circuit diagram of a known control system for such a variable displacement hydraulic pump which is disclosed in Japanese Patent Laid-open Publication No. Heisei. 1-116294. With reference to this drawing, the known control system includes a servo valve 3 or a speed control valve which is displaceable between three positions, a hydraulic feeding position at which a hydraulic fluid under pressure is supplied to a larger chamber 1a of a servo cylinder 10 by way of a charge conduit 2, a neutral position at which the conduit 2 is closed and a drain position at which the hydraulic fluid under pressure is discharged from the larger chamber 1a of the servo cylinder 10 to an oil reservoir (not shown) through the servo valve 3. This servo valve 3 has a servo spool 3a of which one end is linked, using a feedback lever 5, to a servo piston 4 of the servo cylinder 10. In addition, this known control device is provided with a horsepower control pilot ram 7, a flow control pilot ram 8 and a link mechanism comprising two levers 9a and 9b, which are connected to the pistons 7a and 8a of the control pilot rams 7 and 8, respectively. The link mechanism 9a and 9b selects one of the two control pilot rams 7 and 8 which displaces less than the other and causes the feedback lever 5 to actuate in accordance with the displacement of the selected ram 7 or 8. Here, the horsepower control pilot ram 7 has the pilot piston 7a which is displaceable in response to a pump output pressure Pd of a variable displacement pump 6, while the flow control pilot ram 8 has the pilot piston 8a which is displaceable in response to an outside pilot pressure Pi.
In the drawing, the reference numeral 6a denotes a swash plate or an inclined axis of the pump 6 of which the inclination angle is changed in accordance with the displacement of the servo piston 4 of the servo cylinder 10.
However as noted, this type of control system necessarily becomes a complicated three-lever type of system since it has three levers, that is, the feedback lever 5, the horsepower control lever 9a and the flow control lever 9b, in order to simultaneously independently perform the constant horsepower control and the flow rate control. Furthermore, the horsepower control pilot ram 7, having the pilot piston 7a and a spring 7b, and the flow control pilot ram 8, having the pilot piston 8a and a spring 8b, are independently cooperated with the separated levers 9b and 9a of the link mechanism. In result, this type of known control system has a disadvantage in that it has serious problems caused by difficulty of design and preparation thereof and striving to accomplish a desired accuracy. Furthermore, this control system has many link points because it is provided with the three levers 5, 9a and 9b linked to each other as described above and this causes these link points to be necessarily abraded as it is used for a long time, as a result, another problem of this system resides in the pass the possibility of deterioration of control performance of the system due to the accumulated abrasion of the link points.